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From: Robert Watson <rwatson@FreeBSD.org>
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To: Bruce Evans <bde@zeta.org.au>
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cc: Don Lewis <truckman@FreeBSD.org>
cc: current@FreeBSD.org
Subject: Re: CFR: fifo_open()/fifo_close() patch
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On Sat, 17 May 2003, Bruce Evans wrote:

> On Sat, 17 May 2003, Don Lewis wrote:
> 
> > On 17 May, Bruce Evans wrote:
> > > My question is mainly: why do you want or need the extra complexity for
> > > using the vnode interlock here?
> >
> > I want to use the vnode interlock so that I can msleep() on it to avoid
> > a race condition.  If I rely on the vnode lock to protect fi_readers and
> > fi_writers, another thread could sneak in, update them, and call
> > wakeup() between the VOP_UNLOCK() call and the tsleep() call, causing
> > the thread calling tsleep() to miss the wakeup().
> 
> I see. 
> 
> I now think fifo_close() needs both the vnode lock and the interlock. 
> Its socantrcvmore() calls should be atomic with decrementing the
> reader/writer counts to 0.  I think locking them with the interlock
> would work, but this depends too much on their internals (not sleeping). 
> Sorry, I deleted your original patch and don't remember exactly what it
> does here.
> 
> NetBSD changed VOP_CLOSE() to "L L L" 4+ years ago. 

No comment on the remainder, but moving VOP_CLOSE to L L L seems like
quite a reasonable direction to me.

Robert N M Watson             FreeBSD Core Team, TrustedBSD Projects
robert@fledge.watson.org      Network Associates Laboratories